1. Field of the Invention
This invention relates to the production of plastics products.
2. Description of the Prior Art
It is well known to mold plastics products, such as polyethylene and its co-polymers, by injection molding.
However, injection molding suffers from the disadvantage that tooling costs are relatively high and that there is a limited size range capable of injection molding.
It has also been proposed to mold thermoplastic materials by placing powdered or granular material in a hollow metal mold shell which is then suspended in a mounting system enabling rotation in one or two axes causing the powdered material to flow over all the interior surfaces of the mold shell (which have, of course, the appropriate mold impressions). The mounting system and the mold is then contained within an oven which heats the mold to the desired appropriate temperature (above the melting temperature of the plastics polymer concerned) and after a period of rotation with the oven the powdered material melts and creates a relatively even coating over the inside of the mould thus taking the mold impression. The mold is then removed from the oven for cooling and the molded article then removed from the mold.
Such a rotational system does, in certain circumstances, suffer from the drawback that high melt flow (low molecular weight) materials must be used in order to achieve satisfactory mold quality. Such materials, in practice, commonly have poor physical properties unsuitable for certain applications. In addition, materials of low or moderate melt flow give, not only poor molding quality, but also a porosity which further reduces certain physical properties. It is for this reason that the practice of crosslinking during the rotational molding process has become common. The production of high quality, tough moldings can be made possible with crosslinking. There are however associated disadvantages of higher material cost and inability of moldings, now of a thermoset nature, to be fusion welded, for example.
It is an object of the present invention to provide a moldings arrangement which overcomes or at least substantially reduces the above mentioned disadvantages and drawbacks.